The present disclosure relates to a developing device which uses a two-component developer containing a magnetic carrier and a toner and supplies the toner to an image carrier. The present disclosure also relates to an electrophotographic image forming apparatus comprising this developing device.
In an electrophotographic image forming apparatus, light based on image information read from a document image or image information transmitted or otherwise delivered from a computer or another external machine is radiated onto the peripheral surface of an image carrier (a photosensitive drum) on which a photosensitive layer is formed, an electrostatic latent image is formed, toner is supplied to the electrostatic latent image from a developing device to form a toner image, and the toner image is then transferred onto paper. After this transfer process, the paper undergoes a process of fixing the toner image and is then ejected to the exterior.
Recently, the configurations of image forming apparatuses have grown complex along with the advancement of color printing and high-speed processing, and adapting to high-speed processing has necessitated high-speed rotation of the toner stirring member inside the developing device, whereby the internal pressure of the developing device is likely to be a positive pressure higher than atmospheric pressure. When the developing device interior has a positive pressure and toner from the developing device interior is supplied to the photosensitive drum, some of the toner leaks as scattered toner out of an opening (a toner supply hole) in the developing device that faces the photosensitive drum, and the interior of the image-forming apparatus is contaminated.
Particularly, with a developing system that uses a two-component developer containing a magnetic carrier and a toner, and that also uses a magnetic roller (a developer carrier) for carrying the developer and a developing roller (a toner carrier) for carrying only the toner, toner that has not been used in the developing is stripped away from the developing roller by a magnetic brush formed on the magnetic roller in the facing portion of the developing roller and the magnetic roller. Therefore, the toner readily floats in the vicinity of the facing portion of the developing roller and the magnetic roller, and the floating toner leaks out as scattered toner. Floating toner that has accumulated in the interior of the developing device forms clumps and falls down onto the developing roller, disrupting the thin layer of toner on the developing roller, thereby readily causing problems such as “dropping toner,” in which toner is not supplied to the portion where toner should be adhering in the peripheral surface of the photosensitive drum.
A known technique for resolving problems such as the one described above involves forcefully drawing in the air in the gap between the toner carrier and a covering member for covering part of the external peripheral surface of the toner carrier, whereby toner scattering and heat generation in the developing device are suppressed even if the developing step carried out at a higher speed.
However, this developing apparatus is a developing system that supplies the photosensitive drum with the toner in the two-component developer supplied onto the developing roller from a stirring screw and that does not have a developer carrier and a toner carrier. The portion where positive pressure arises in the developing device differs depending on whether or not there is a developer carrier, as does the extend of the positive pressure arising.
Consequently, with a method of forcefully drawing in the air in the gap between the toner carrier and the covering member for covering part of the external peripheral surface of the toner carrier as described above, it has not been possible to effectively resolve the problem of floating toner in the space enclosed by the developer carrier, the toner carrier, and the inner surface of the developing container, which is a phenomenon unique to developing systems that have a developer carrier and a toner carrier. Because the above method uses a configuration in which an airflow including the floating toner passes over an restricting blade, there is a risk of dropping toner leading to image flaws when floating toner accumulates on the blade and the accumulated toner aggregates and adheres to the toner carrier.
Also known is a developing device in which the wall of the developing container that faces below the toner carrier is provided with an airflow inlet for taking in air from outside the developing device, whereby an airflow flowing from the outside of the developing container to the inside flows into the negative-pressure vicinity of the restricting blade, and the incoming airflow causes the floating toner in the vicinity of the restricting blade to be carried by the rising airflow, thereby preventing the accumulation of toner on the restricting blade.
According to this method, the flow of air in from the airflow inlet provided to the wall of the developing container that faces below the toner carrier makes it possible to reduce the pressure of the portion that has positive pressure due to the rotation of the developer carrier and the toner carrier and to suppress the leaking of developer. The incoming airflow is joined with the rising airflow, whereby the floating toner in the vicinity of the restricting blade is carried by the rising airflow and the accumulation of toner on the restricting blade can be prevented.
However, despite the inclusion of the airflow inlet, when the speed of the image forming apparatus main body is increased and there is a large amount of floating toner, it has been difficult for the floating toner to be sufficiently carried by the airflow created by the rotation of the developer carrier and the toner carrier, and it has also been difficult for the accumulation of toner to be sufficiently prevented. With a configuration having an airflow inlet provided to the wall of the developing container that faces below the toner carrier, although it is possible to resolve the problem of floating toner in the vicinity of the restricting blade positioned below the facing portion of the developer carrier and the toner carrier, it has not been possible to effectively prevent the external leaking of floating toner that occurs in the space enclosed by the developer carrier, the toner carrier, and the top surface of the developing container.
Furthermore, also known is an image forming apparatus in which a through-hole for taking in air from outside of the developing device is provided in the wall of the developing container that faces below the toner carrier, and an air ejection hole communicated with the duct is provided in the top end of the developing container above the border between the toner carrier and the developer carrier.
According to this method, airflows from the through-hole of the developing container that faces below the toner carrier to the duct via the air ejection hole provided in the top end of the developing container, whereby floating toner present in the space enclosed by the developer carrier, the toner carrier, and the inner surface of the developing container can be effectively ejected even when there is a large amount of floating toner.
However, with this method, when all of the floating toner present is drawn into the duct from the air ejection hole, there have been problems with the toner accumulating in the duct, clogging becoming severe in the filter attached to the ejection hole side of the duct and leading to numerous filter replacements, and the like. When drawing the floating toner into the duct is insufficient, there has been a risk of the floating toner leaking out to the exterior through the opening in the developing device, and toner contamination occurring in the interior of the image forming apparatus.